This invention relates to poles for thin film recording heads.
As the track density is increased in magnetic recording, it is necessary to reduce the track width and the width of the head. Reducing the width of the head results in "edge closure domains" in single layer poles when the pole width is reduced to a critical value where the edge domains dominate. These edge domains can conduct flux only by domain wall motion. This process fails at high frequencies and leads to "Barkausen noise" at low frequencies. This situation is illustrated in FIGS. 1a-1d. In FIG. 1a a pole 10 includes horizontally oriented normal domains 12 and edge domains 14. As shown in FIG. 1b, flux is conducted by rotation. As the pole width is reduced to a critical value, the edge domains touch as shown in FIG. 1c. The magnetic permeability becomes low at high frequencies and flux is conducted by wall motion. As shown in FIG. 1d, Barkhausen noise is created when a wall 16 encounters a defect 18 and becomes temporarily stuck.
In addition to the single layer poles illustrated in FIGS. 1a-1d, laminated ferromagnetic structures are known. Those structures related to narrow track applications usually involve relatively thick insulators (e.g. 200 Angstroms) alternating with a large number of thin (700 Angstroms) ferromagnetic films. A structure including two ferromagnetic layers separated by a very thin (less than 100 Angstrom) layer of nonmagnetic material is disclosed in "Coercivity, Structure, and Stoichiometry of Permalloy/Alumina Multilayers" by N. J. Jubb et al. presented at the 1984 M.M.M. Conference, and "Sensitivity of Single Sided Single Pole Type Perpendicular Magnetic Head" by Iaso Watanabe et al. in Research Inst. of Ele. Eng., Tohoko University. The Watanabe paper discusses narrow tracks for high frequency operation. In "Reproducing Sensitivity of Single-Pole-Type Head with Co--Zr--Nb/SiO.sub.2 Multilayered Films" by S. Iwasaki, Y. Nakamura, I. Watanabe, K. Wamakawa, H. Hasegawa, IEEE Translation Journal on Magnetics in Japan, Vol. TJMJ-2, No. 5, May 1987, p. 386, heads are disclosed including ferromagnetic layers separated by a nonmagnetic layer. The ferromagnetic layers were of the same material and had equivalent uniaxial anisotropy.